Asphalt aggregate hot mix that is used for paving is composed of aggregate particles of various sizes, such as sand and crushed rock together with an asphaltic binder. The material is usually mixed in a drum mixer or a pug mill and is then stored more or less temporarily in a thermally insulated storage bin pending its transport to a site where it is to be used. The storage bin is a relatively tall silo-like structure that is mounted on legs which support its bottom outlet at a high enough elevation so that a truck can be driven under it for loading. The mixed material is fed into an inlet in the top of the storage bin by means of an elevator to which the pug mill or mixing drum discharges.
In recent years the type of elevator that has come to be preferred for asphalt mix storage plants is a bucket elevator that extends straight up alongside the storage bin. Since the top of the elevator is laterally spaced from the storage bin inlet, the elevator extends up to a level substantially above that of the inlet, and asphalt mix is carried across the intervening portion of the storage bin by gravity, in an inclined chute that extends obliquely downwardly from the elevator to the inlet.
Although the bucket elevator is superior in most respects to the slat conveyor formerly most commonly used for filling asphalt mix storage bins, one disadvantage that has appeared with bucket elevator installations is a tendency towards segregation of the components of the asphalt mix. A satisfactory paving material is a very uniform mixture of fines, coarse components and asphaltic binder. Heretofore, however, when asphalt mix material was charged into a storage bin from a bucket elevator, the larger components of the mix tended to settle out close to the cylindrical wall of the storage bin, sometimes predominantly at one side of the bin, while the fines tended to remain in the middle of the bin. Once such segregation had occurred, the material remained segregated, since little or no mixing takes place during discharge of the material from the storage bin into a truck.
Segregation of mix materials has been a long standing problem in connection with storage bins for asphalt aggregate mix. One expedient now generally used to prevent or minimize segregation is the provision of a hopper in the upper portion of the storage bin into which the incoming material is initially charged. Until the hopper is substantially filled, its bottom outlet remains closed, but then the outlet is abruptly and briefly opened to dump the entire contents of the hopper batchwise into the storage bin proper. If the mix material were run directly into the storage bin at a more or less steady rate, a cone of fines would build up in the center of the bin, and coarser material would roll down the flanks of the cone and settle along the wall. When the storage bin is filled by dumping material batchwise into it from a hopper, the build-up of such a cone is prevented, but filling the bin in this manner cannot effect a remixing of the materials if segregation has already occurred during charging of material into the hopper.
Bucket elevators have been in use for several years in connection with asphalt mix storage apparatus, and during that time it has come to be generally recognized that a bucket elevator induces a segregation problem even when the storage bin is equipped with a batch charging hopper; but no solution to that problem has heretofore been available.
The present invention is based upon a recognition that a substantial amount of segregation at the charging hopper is promoted by the chute along which the material moves by gravity from the elevator to the storage bin inlet. The fines tend to slide along the bottom of that chute and to be impeded by friction, whereas the larger components tend to move along the chute more freely and therefore have substantially more forward inertia as they fall from the chute into the hopper. As a result, the coarser components of the mix tend to accumulate at one side of the hopper while the fines accumulate at its other side, and, as pointed out above, once such segregation has occurred, it persists all the way to the site of use.
It has also been found that a certain amount of segregation takes place as the mix is charged into the bucket elevator, inasmuch as the material is transferred from the mixing apparatus to the elevator by means of a chute along which it flows by gravity. In this case, too, the greater forward inertia of the coarse components tends to separate them from the fines, so that the material is already segregated in the elevator buckets.